July 16, 2008

Preliminary design for Strathcona Dam proposes underwater seismic isolation

JEAN SORENSEN

correspondent

Vancouver’s heritage city hall isn’t the only structure that is under consideration for an innovative seismic retrofit that essentially uses disc-shaped, rubber shock absorbers to isolate the effects of an earth-quake.

A proposal for BC Hydro’s intake tower at the Strathcona Dam on Vancouver has moved to the preliminary design stage, said John Sherstobitoff, with Sandwell Engineering Inc., who has been working on the project for the past two and half years.

The Strathcona Dam project was built between 1955-1958 and the proposal for upgrading the tower using this new method is seen as both challenging and record-breaking.

Sherstobitoff said the design calls for the use of a method known as seismic isolation of a structure that is partially underwater.

“We will be slicing into the building and doing the isolation under water,” he said, adding this is the first project of this kind in the world which will involve placing the rubber discs underwater.

A decision to go ahead with the seismic upgrade is expected to be made early next year by BC Hydro.

Seismic isolation consists of placing a series of one to one-and-half-metre wide discs within the building’s supports or foundation creating a buffer with the earth.

These discs then act like shock absorber on a vehicle, Sherstobitoff said.

Isolation is a new concept for buildings, but it is a technique that is commonly used to stabilize bridges, usually involving bearings and expansion joints, which compensate for movement.

The same concept has been adapted for other structures.

Sherstobitoff has been advocating its use to clients and was instrumental in bringing it to the attention of the City of Vancouver as a viable means seismically upgrading the heritage structure.

As well, currently several other B.C. clients are interested in the concept, but do not want to go public.

Sandwell has been teaming with two U.S. firms, both known for their expertise in seismic isolation. They are Seismic Isolation Engineering Inc. (SIE) in Emeryville, California and Forell/Elsesser Engineering based in San Francisco.

SIE’s Dr. Cameron Black, an Albertan, said the process being proposed for the city hall is common for older structures.

“It’s been most applicable for retrofitting heritage buildings,” he said, adding there is limited impact on the architectural uniqueness of these structures.

But, it is also used in newer commercial or industrial buildings in the U.S., where there is a concern that an earthquake may result in loss of valuable production for specialized industries.

Black said the weight of the building keeps it where it is, but the earth below is softer and moves. These rubber mats are a buffer between the rigid building and soft earth.

“It lets the earth move without the building coming along with it,” he said.

The Vancouver city hall is not the first civic structure that SIE and Forell/Elsesser have been involved with.

Others that have undergone seismic isolation include the Los Angeles, San Francisco and Oakland city halls.

“We have contributed in some way,” Black said, either through a peer review, working with the product manufacturers or consulting.

Seismic isolation is becoming more common in the U.S., said Black, but added that it was popularized in Japan. Today’s Japanese condominium towers being marketed use seismic isolation as a sales tool.

Its use grew substantially after the 1995 Kobe earthquake.

“In the three-year period prior to the 1995 earthquake, 15 seismic isolated buildings were licensed for construction. In the three years following the earthquake, 450 isolated buildings were approved,” states an impact paper available on SIE’s website.

Garrick Bradshaw, Vancouver city hall’s director of facility design and management, said the city is concerned about preserving the hall’s unique design features in the event of a quake.

The damage sustained to a rigid structure comes from the whipping of the building as the first tremors are carried into the building causing movement in one direction and subsequent tremors move the building in a different direction, he said.

Bradshaw said the city is at an investigative stage only and are learning more about the concept and considering it as an option.

“We are nowhere near a decision yet and it has not gone to council,” he said.

Vancouver’s city hall opened in 1936 and was built for $1 million during the Depression, when it was used as a make-work project, as well as a beacon for an emerging city.

Its design by Townley & Matheson Architects is hailed as a transition between the glitzy Art Deco period and the simplistic elegance seen in the emergent horizontal Moderne design.

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